<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        // 赋值运算符  =   +=  -=  *=  /=  %=
        // 
        var a  =  20;

        // +=
        a+=20;//
        console.log(a);//40
    </script>
</body>
</html>